1. Field of Invention
This invention relates to wavelength-converted light sources, such as wavelength-converted semiconductor light emitting devices.
2. Description of Related Art
Semiconductor light-emitting devices including light emitting diodes (LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavity laser diodes (VCSELs), and edge emitting lasers are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, III-nitride, or other suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The stack often includes one or more n-type layers doped with, for example, Si, formed over the substrate, one or more light emitting layers in an active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region. Electrical contacts are formed on the n- and p-type regions.
Since the light emitted by current commercially available III-nitride devices is generally on the shorter wavelength end of the visible spectrum, the light generated by III-nitride devices can be readily converted to produce light having a longer wavelength. It is well known in the art that light having a first peak wavelength (the “primary light”) can be converted into light having one or more longer peak wavelengths (the “secondary light”) using a process known as luminescence/fluorescence. The fluorescent process involves absorbing the primary light by a wavelength-converting material such as a phosphor and exciting the luminescent centers of the phosphor material, which emit the secondary light. The peak wavelength of the secondary light will depend on the phosphor material. The type of phosphor material can be chosen to yield secondary light having a particular peak wavelength.
FIG. 1 illustrates a prior art phosphor-converted LED 10 described in U.S. Pat. No. 6,351,069. The LED 10 includes a III-nitride die 12 that generates blue primary light when activated. The III-nitride die 12 is positioned on a reflector cup lead frame 14 and is electrically coupled to leads 16 and 18. The leads 16 and 18 conduct electrical power to the III-nitride die 12. The III-nitride die 12 is covered by a layer 20, often a transparent resin, which includes wavelength-converting material 22. The type of wavelength-converting material utilized to form the layer 20 can vary, depending upon the desired spectral distribution of the secondary light that will be generated by the fluorescent material 22. The III-nitride die 12 and the fluorescent layer 20 are encapsulated by a lens 24. The lens 24 is typically made of a transparent epoxy or silicone.
In operation, electrical power is supplied to the III-nitride die 12 to activate the die. When activated, die 12 emits the primary light away from the top surface of the die. A portion of the emitted primary light is absorbed by the wavelength-converting material 22 in the layer 20. The wavelength-converting material 22 then emits secondary light, i.e., the converted light having a longer peak wavelength, in response to absorption of the primary light. The remaining unabsorbed portion of the emitted primary light is transmitted through the wavelength-converting layer, along with the secondary light. The lens 24 directs the unabsorbed primary light and the secondary light in a general direction indicated by arrow 26 as output light. Thus, the output light is a composite light that is composed of the primary light emitted from die 12 and the secondary light emitted from the wavelength-converting layer 20. The wavelength-converting material may also be configured such that very little or none of the primary light escapes the device, as in the case of a die that emits UV primary light combined with one or more wavelength-converting materials that emit visible secondary light.
Alternative configurations of phosphor-converted LEDs include LED devices grown on single crystal luminescent substrates as described in U.S. Pat. No. 6,630,691, thin film phosphor layers formed on LEDs as described in U.S. Pat. No. 6,696,703, and conformal layers deposited on LEDs by electrophoretic deposition as described in U.S. Pat. No. 6,576,488 or by stenciling as described in U.S. Pat. No. 6,650,044.
The above described devices, where the phosphor layer is formed on a surface of the light emitting device, may have several disadvantages. It is difficult to achieve color uniformity when a phosphor is applied directly to a surface of an LED, due to variations in the path of light through the phosphor and in the thickness of the phosphor layer. Also, heat from the LED can undesirably shift the color point of the phosphor or degrade the phosphor.